Edge connectors for printed circuit boards comprising conductive ink

ABSTRACT

Disclosed are edge contacts for printed circuit boards comprising a copper lead coated with conductive ink comprising a binder and graphite powder, carbon black, and silver flakes. Also disclosed are printed circuit boards containing the contacts and methods for manufacturing the contacts and circuit boards.

This is a continuation of application Ser. No. 09/596,666, filed Jun.19, 2000 now abandoned, which is a divisional of application Ser. No.09/197,685, filed Nov. 20, 1998, now U.S. Pat. No. 6,265,051.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a printed circuit boards and, morespecifically, to edge connectors disposed at the periphery of printedcircuit boards.

2. Description of the Related Art

Printed circuit boards of the type used in various computer hardware,e.g., modems, commonly include a copper conductor over which is applieda layer of nickel metal followed by a layer of gold. Because of thecomplexity of applying a layer of gold over a layer of nickel on acopper conductor, and the associated relatively high cost of gold, thereexists a need for a replacement for the gold and nickel materials inedge connectors on printed circuit boards.

The blending of resins or adhesives with powdered conductive materialssuch as, gold, silver, copper, nickel or other metals or alloys,provides conductive pastes or inks. Such materials, also known aspolymer thick films (ptf), generally fall into two classes. One class isthe thermosetting kind (preferred for permanence) of which the epoxys,acrylics, and polyesters are examples. The other class is thethermoplastic kind (which soften when heated) composed of synthetic ornatural polymer, (such as, polyimide siloxane, nylon, neoprene, orrubber) in a solvent or other suitable carrier.

For further information pertaining to conductive inks, conductiveadhesive inks, adhesives and surface mount technology, in general, thereader is directed to Radio-Electronics, page 59 et seq., November,1987, “Introduction to SMT”, by Forrest M.

Conductive inks typically include metals such as silver, copper, lead ortin to provide electrical conductivity together with polymeric binderswhich provide adhesion.

Conductive inks have been applied by screen printing onto non-moldableinsulating baseboard substances, such as FR4 or glass epoxy.

However, when printed circuit boards are manufactured by applying aconductive paste such as copper paste on the insulating base board byscreen printing, there is generally inadequate adhesion of theconductive paste or ink to the circuit terminal portion. This isparticularly pervasive when heat is applied to the board during thesoldering of electrical components to the terminal portion.

Also, specific levels of resistivity are required for use as edgeconnectors in Peripheral Component Interconnect (PCI), Industry StandardArchitecture (ISA), and Audio Modem Riser (AMR) buses.

SUMMARY OF THE INVENTION

The present invention provides edge connectors or edge contacts onprinted circuit boards that overcome the disadvantages described above.

More specifically, the invention provides edge connectors or contacts inwhich the gold and nickel layers are replaced with a single layer of aconductive ink. The use of the conductive ink as an edge connector asdescribed herein makes unnecessary the use of nickel plating between theupper layer of the connector and the copper conductor. Thus, not only isthe use of gold avoided, but also the use of nickel, making the processmore efficient and less expensive.

Thus, in one aspect, the invention provides edge connectors on a printedcircuit board that are substantially permanent and do not crack orpeal-off of the circuit board.

In another aspect, the invention provides printed circuit boards havingat least one edge connector, where the edge connector comprises a copperconductor coated with a conductive ink where the ink generally has asheet resistivity of about 0.1 to 0.5 Ω/square/15 μm.

In a further aspect, the invention provides printed circuit boardshaving one or a plurality of edge connectors in which a copper conductoris coated with a conductive ink where the ink has a contact resistanceof about 10×10⁻³Ω to 70×10⁻³Ω.

The conductive ink suitable for use herein is a thermosetting resincontaining graphite powder, carbon black and silver flakes. The silverloading is generally in the range of from about 10-50% by weight of thewet ink composition. The silver flakes suitable for use herein have agrind size not greater than about 10 μm. Viewed another way, the silverloading is from about 50-75% by volume of the ink composition. Such anamount is sufficient to provide the edge contact with a sheetresistivity of from about 0.1 Ω/sq/15 μm to 0.5 Ω/sq/15 μm and a contactresistance of from about 10×10⁻³ to 70×10⁻³Ω.

While the inventive connector is particularly suitable for use inmanufacturing ISA bus edge connectors, it may also be used inmanufacturing PCI and AMR bus edge connectors.

The invention also provides methods for forming edge connectors onprinted circuit boards comprising applying a conductive ink having asheet resistivity of about 0.1 to about 0.5 Ω/sq/15 μm over a copperconductor terminating at the edge of the circuit board. Generally, theconductive inks are applied to the circuit board using screen printingor other suitable technologies known to those skilled in the art.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the terms “ink” and “paint” are meant to be synonymousand each are intended herein to mean and include the other. Similarly,the terms “printing” and “painting” are also meant to be synonymous.Screen printing (or more accurately, polyester screening or stainlessscreening) is one method of “inking” or “painting”.

The edge connectors of the invention comprise a copper conductordisposed on a printed circuit board where the copper is coated withconductive ink. The inks suitable for use herein comprise athermosetting resin containing graphite powder, carbon black, and silverflakes. The printed circuit boards are preferably PCI, ISA, or AMR(Audio Modem Riser) buses. Those skilled in the art will recognizematerials having characteristics similar to the carbon black andgraphite powder that may be used in place of the carbon black andgraphite powder without altering the required properties of theconductive ink.

The conductive ink coated on the edge connector, after application tothe printed circuit board and curing, preferably has a sheet resistivityof from about 0.1-0.5 Ω/square/15 μm, more preferably about 0.2-0.3Ω/square/15 μm, and most preferably about 0.25 Ω/square/15 μm. Suchresistivity provides electrical properties similar to those ofconventional edge contacts.

In the printed circuit boards of the invention, the resistance acrossthe edge connector is from about 10×10⁻³Ω to 70×10⁻³Ω. Preferably, theedge connector has a contact resistance of about 50×10⁻³Ω, morepreferably about 35×10⁻³Ω, and most preferably about 30×10⁻³Ω.

The method of manufacturing edge connectors on a printed circuit boardaccording to the invention comprises applying a conductive ink over acopper conductor terminating at an edge of the circuit board. The curedink generally has a sheet resistivity of from about 0.1 to 0.5 Ω/sq/15μm. More preferably, the sheet resistivity is from about 0.2 to 0.3Ω/sq/15 μm, and most preferably about 0.25 Ω/sq/15 m.

The binder or resin is preferably an epoxy resin comprising phenolicmonomers.

The conductive ink may be coated on the substrate using any of a varietyof methods known in the art. The preferred method is screen printingaccording to well known procedures. Other methods include brush, roller,spraying, dipping, masking, vacuum plating, vacuum deposition or anycombination of the foregoing. Generally, the methods include a heatcuring step or curing in a vapor phase oven or with infrared (IR) orultraviolet (UV) radiation to form a continuous electrical pathway.

More specifically, the method for applying the ink comprises firstmaking a printing screen or stencil containing the pattern of thedesired edge contacts. The processes and apparatus for screen printingand stencil printing are well known to those skilled in the art. Thescreen is then used in a screen printing apparatus to print multiplecopies of the printed circuit on the substrate selected. Such substratesinclude polyimide, phenolic, epoxy, BT-epoxy, cyanate ester, or otherthermoset laminates. The substrates may be flexible or, preferably,rigid. Alternatively, the substrate may be injection molded or extrudedsheet of polyetherimide, polyethersulfone, polyetherketone,polyarylsulfone, liquid crystal polymer, or other thermoplastic. Othersubstrates may also be used, including ceramics, insulated metal sheets,glass, quartz, graphite or any other material to which the compositioncan be adhered and which can withstand the curing temperature.

After printing the uncured composition in the pattern of the desiredprinted circuit on the substrate, the composition is then cured byapplication of heat. A static oven may be employed. The heat source maybe infrared, panel heater, convection, forced hot air, induction, vaporphase condensation, microwave, etc. Such heat sources and ovens are wellknown to those skilled in the art. A conveyorized system having multiplestages may be used to heat, dry, cure, and then cool the composition ina controlled way. If needed, components may be soldered to the printedcircuit with a solder wave, soldering iron, hot air gun, or solder pastereflow, all common techniques well known in the art. Alternatively,components may be adhered to the printed circuit using a conductive ink.The inks suitable for adhering components the board may be the inks ofthe invention or other readily available inks.

The printed or cured conductive inks on the edge contacts have athickness of approximately 15 μm.

The composition of the ink includes a thermosetting or radiation curablebinder which may optionally be thinned with a thinner, as well as theelectrically conductive particles, preferably a mixture of silver flakeand carbon black. Optionally, the composition may include an agent,e.g., fumed silica, to modify the thixotropic characteristics of theink, preferably. The relative proportions of these components,especially the binder and the thinner, encourage wetting of the ink tothe surface of the substrate and a surface tension that leaves arelatively thin, smooth film of ink adhering to the surface of thecircuit board.

The thickness of film is sufficient that when the film cures, itprovides the necessary conductivity. However, the film is not so thickthat the resulting layer of ink will separate from the substrate orrestrict the insertion of the board into an appropriate receptacle.

While various conductive inks may be used to achieve the requisitedurability when the ink is cured, suitable conductivity across theconnector is achieved using the proper loading of appropriately sizedsilver flakes. Such flakes have a grind size not greater than about 10μm and comprise about 10-50% by weight of the ink. A preferred amount ofsilver flakes in the composition is from about 35-50%, more preferablyfrom about 40-50%, by weight of the ink composition. The amount ofsilver flakes suitable for use herein, based on the volume of the inkcomposition, is from about 50 to 75%.

Other inks that may be used herein include copper or silver basedcompositions containing silver oxide or tin and lead alloy bindingcompositions.

Optional components suitable for use in the preferred inks hereininclude copper, lead, tin and other metals.

The preferred resin is one that is heat curable, i.e., a thermosettingresin. Presently preferred is a phenolic epoxy resin.

The preferred ink for use herein comprises, by weight of the ink,

(a) from about 10 to 40% of an epoxy resin comprising phenolic monomers;

(b) from about 3 to 10% of carbon black;

(c) from about 8 to 20% of graphite powder;

(d) from about 10 to 50% of silver flakes having an average grind sizenot greater than about 10 μm.

More preferred compositions of the ink comprise, by weight of the ink,

(a) from about 10 to 40% of an epoxy resin comprising phenolic monomers;

(b) from about 5 to 10% of carbon black;

(c) from about 12 to 20% of graphite powder;

(d) from about 10 to 50% of the silver flakes;

(e) up to about 30% thinner, preferably Diethylene glycol (commerciallyavailable from Union Carbide as Carbitol®), Ethylene glycol monobutylether acetate (commercially available from Union Carbide as ButylCellosolve® Acetate), or 2-(2-Butoxyethoxy)ethanol (commerciallyavailable from Union Carbide as Butyl Carbitol®); and

(f) up to about 8% methanol.

Particularly preferred compositions of the ink comprise, by weight ofthe ink,

(a) from about 10 to 40% of an epoxy resin comprising phenolic monomers;

(b) from about 3 to 8% of carbon black;

(c) from about 8 to 18% of graphite powder;

(d) from about 10 to 55% of the silver flakes;

(e) from about 10-35% thinner, and

(f) from about 4-8% methanol.

Variations in the above ink composition are possible; in preferredcompositions, the quantities of binder and thinner generally will beapproximately equal. However, in certain embodiments, the amount ofthinner may be as low as about 1% by weight. Examples of preferredthinners are Carbitol®, Butyl Cellosolve® Acetate, or Butyl Carbitol®.

The carbon black within the ink composition tends to bridge gaps betweenthe silver flakes, creating electrical continuity between the individualflakes of silver and reducing the possibility of electrical voidsbetween the individual silver flakes.

Until fully cured, the electrical conductivity of the film of ink hasnot reached its highest value. Increased conductivity is achieved byshrinkage of the ink upon curing which brings conductive particles intomore substantial contact. Curing occurs as a result of removal ofthinner from the composition and the resulting action of the binder.

In a highly preferred embodiment, the conductive ink or ptf isElectrador® 5601 Carbon Conductor Paste, commercially available fromElectra Polymers and Chemicals.

The edge contacts of the invention may be manufactured according to thefollowing procedure.

Brush or chemically clean copper surfaces to provide a waterbreak-freesurface.

Screen printing settings:

Screen mesh: about 200 mesh stainless steel about 145 mesh polyesterminimum mesh opening about 39% Squeegee: about 70 to 80 Shore Emulsion:about 1-1.5 mil (25 − 28μ) Cure schedule: convection oven: about 45minutes at          about 165° C. IR tunnel oven: about 5 minutes at         about 165° C.

The invention and the manner and process of making and using it, are nowdescribed in such full, clear, concise and exact terms as to enable anyperson skilled in the art to which it pertains, to make and use thesame. It is to be understood that the foregoing describes preferredembodiments of the present invention and that modifications may be madetherein without departing from the spirit or scope of the presentinvention as set forth in the claims. To particularly point out anddistinctly claim the subject matter regarded as invention, the followingclaims conclude this specification.

What is claimed is:
 1. A method of manufacturing edge connectors on aprinted circuit board where the maximum resistance across the edgeconnector is about 70×10⁻³Ω comprising applying a conductive ink thatprovides a sheet resistivity of about 0.1 to 0.5 Ω/sq/15 μm over acopper conductor terminating at an edge of the circuit board, theconductive ink comprising a thermoset epoxy binder, graphite powder,carbon black, and silver flakes, wherein the silver flakes have anaverage size not greater than about 10 μm.
 2. A method according toclaim 1, wherein the sheet resistivity is about 0.2 to 0.3 Ω/sq/15 μm.3. A method according to claim 1, wherein the sheet resistivity is about0.25 Ω/sq/15 μm.
 4. A method according to claim 3, wherein the thermosetepoxy binder comprises an epoxy resin containing phenolic monomers.
 5. Amethod according to claim 4, where the ink further comprises methanoland diethylene glycol.
 6. A method according to claim 5, wherein the inkcomprises (a) from about 20 to 40% of an epoxy resin comprising phenolicmonomers; (b) from about 3 to 10% of carbon black; (c) from about 8 to20% of graphite powder; (d) from about 10 to 50% of silver flakes havingan average grind size not greater than about 10 μm.
 7. A methodaccording to claim 5, where the ink comprises (a) from about 20 to 40%of an epoxy resin comprising phenolic monomers; (b) from about 3 to 10%of carbon black; (c) from about 8 to 20% of graphite powder; (d) fromabout 10 to 50% of the silver flakes having an average grind size notgreater than about 10 μm; (e) up to about 30% thinner and (f) up toabout 8% methanol.
 8. The method of claim 1 wherein the conductive inkprovides a contact resistance of about 50×10⁻³Ω.
 9. The method of claim2, wherein the edge connector has a contact resistance of about35×10⁻³Ω.
 10. The method of claim 3 wherein the conductive ink providesa contact resistance of about 30×10⁻³Ω.